Temperature compensation for thin film transistors in digital X-ray detectors

ABSTRACT

A digital radiographic detector uses predetermined calibration information corresponding to a first operating temperature of the detector. The calibration data is accessible by the detector to compensate a radiographic image captured by the detector at a second operating temperature different than the first operating temperature. The operating temperature of the detector is monitored at approximately the time at which the radiographic image is captured at the second temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/463,061, filed Aug. 19, 2014, in the name of Timothy J. Tredwell,entitled TEMPERATURE COMPENSATION FOR THIN FILM TRANSISTORS IN DIGITALX-RAY DETECTORS, which claims the benefit of U.S. Provisionalapplication U.S. Ser. No. 61/868,219, provisionally filed on Aug. 21,2013, entitled “COMPENSATION FOR TEMPERATURE DEPENDENCE OF TFTCHARACTERISTICS IN DIGITAL X-RAY DETECTORS”, in the name of Timothy J.Tredwell, which is hereby incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to diagnostic imaging, inparticular, to radiographic imaging using a digital radiographicdetector.

Stationary radiographic imaging equipment may be employed to capturex-ray images on an x-ray detector. Mobile radiographic imagingequipment, such as on rolling carts, may also include an x-ray sourceand a digital x-ray detector to capture x-ray images. The x-ray imagesmay be acquired and stored using various technologies such as computedradiography (CR) and digital radiography (DR).

A digital radiography (DR) detector acquires image data from ascintillating medium using an array of individual photosensors, arrangedin a row-by-column two dimensional matrix, in which each photosensorprovides a single pixel of image data. Each pixel generally includes thephotosensor and a switching element that may be formed in a common planeof a non-single crystal substrate, for example, hydrogenated amorphoussilicon (a-Si:H), to construct the photodiode (photosensor) and athin-film transistor (switching element) in each pixel. In oneembodiment, a front plane of a digital x-ray detector includes a twodimensional array of photosensitive elements (pixels), and a backplaneof the detector, which is electrically connected to the front plane,includes an array of thin-film transistor (TFT) switches each connectedto one of the pixels.

However, there is a need for improvements in the consistency and/orquality of radiographic images, particularly when the images areobtained by an x-ray apparatus designed to use an amorphous siliconbased DR detector.

SUMMARY OF THE INVENTION

An aspect of the present disclosure provides a method and apparatus toaddress disadvantages caused by the use of portable (e.g., wireless)digital radiography (DR) detectors used in mobile radiographic imagingequipment. In one embodiment, a method of operating a digitalradiographic detector is disclosed wherein numerical calibration data isdetermined in association with the detector corresponding to a firstoperating temperature of the detector. The calibration data is stored inan electronic table accessible by the detector, and is configured tocompensate a radiographic image captured by the detector at a secondoperating temperature different than the first operating temperature.The operating temperature of the detector is monitored at approximatelythe time at which the radiographic image is captured at the secondtemperature. In accordance with the magnitude of the second temperature,as monitored, the stored table of the calibration data is modified by acommon factor, which may either increase or decrease the storedcalibration data.

In another embodiment, method to determine gain correction for a digitalradiographic detector is disclosed. The detector includes an array ofpixels forming rows and columns configured to generate a radiographicimage based upon radiation impacting the pixels. A plurality of gatelines and data lines is provided, wherein each one of the gate lines iscoupled to a respective row of pixels and each one of the data linescoupled to a respective column of pixels, and each of the pixelscomprise a thin-film transistor (TFT) connected to a photosensor. Themethod comprises determining a gain correction map for the array ofpixels at a first prescribed temperature and at a predetermined set ofarray operating parameters. By measuring the operating temperature, orat least one temperature sensitive parameter, prior to generating theradiographic image, one or more of the array operating parameters isadjusted in accordance with the temperature to achieve a gain at theoperating temperature or at the at least one temperature sensitiveparameter substantially equal to a gain at the first prescribedtemperature.

In another embodiment, a digital radiography detector for an imagingsystem is disclosed. The digital radiography detector comprises an arrayof pixels, forming rows and columns, configured to generate signalsbased upon radiation impacting the detector. A plurality of scan linesare each coupled to a respective row of pixels and each one of the datalines is coupled to a respective column of pixels. A readout circuitselectively couples the rows of pixels to the respective scan lines andcolumns of pixels to the respective data lines for read out of thesignals. A readout time for the array of pixels is adjusted based on atemperature dependent charge transfer time constant τ_(RC) of athin-film transistor in each of the pixels which is configured toconnect a photosensor of the pixel to a corresponding data line.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the invention.Other desirable objectives and advantages inherently achieved by thedisclosed invention may occur or become apparent to those skilled in theart. The preceding brief description of the invention is intended onlyto provide a brief overview of subject matter disclosed herein accordingto one or more illustrative embodiments, and does not serve as a guideto interpreting the claims or to define or limit the scope of theinvention. The invention is defined only by the appended claims andtheir equivalents. This brief description is provided to introduce anillustrative selection of concepts in a simplified form that are furtherdescribed below in the detailed description. This brief description isnot intended to identify required features or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. The claimed subjectmatter is not limited to implementations that solve any or alldisadvantages noted in this background.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features of the invention can beunderstood, a detailed description of the invention may be had byreference to certain embodiments, some of which are illustrated in theaccompanying drawings. It is to be noted, however, that the drawingsillustrate only certain embodiments of this invention and are thereforenot to be considered limiting of its scope, for the scope of theinvention encompasses other equally effective embodiments. The drawingsare not necessarily to scale, emphasis generally being placed uponillustrating the features of certain embodiments of the invention. Inthe drawings, like numerals are used to indicate like parts throughoutthe various views. Thus, for further understanding of the invention,reference can be made to the following detailed description, read inconnection with the drawings in which:

FIG. 1 is a diagram of an exemplary radiographic imaging system;

FIG. 2 is a schematic diagram of an imaging array for a radiographicdetector;

FIG. 3A shows a perspective view of a portable wireless DR detector;

FIG. 3B is a cross-section of a portion of the portable wireless DRdetector of FIG. 3A along section line A-A; and

FIG. 4 shows an exemplary temperature calibration data table usable withthe exemplary radiographic imaging system disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

The following is a description of exemplary embodiments of theinvention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several figures.

Where they are used, the terms “first”, “second”, and so on, do notnecessarily denote any ordinal or priority relation, but may be used formore clearly distinguishing one element or time interval from another.

FIG. 1 is a perspective view of a digital radiographic (DR) imagingsystem 10 that includes a generally planar digital radiographic detector40, an x-ray source 14, and a digital image monitor 26, according to oneembodiment. The digital detector 40 may include a two dimensional array12 of detector cells 22 (photosensors), arranged in electronicallyidentifiable rows and columns. The digital detector 40 may be positionedto receive x-rays 16 passing through a subject 20 during a radiographicexposure, or radiographic pulse emitted by the x-ray source 14. As shownin FIG. 1, the radiographic imaging system 10 may use an x-ray source 14that emits collimated x-rays 16, e.g. an x-ray beam, selectively aimedat and passing through an area 18 of the subject 20. The x-ray beam 16may be attenuated by varying degrees along its plurality of raysaccording to the internal structure of the subject 20, which attenuatedrays are detected by the array 12 of photosensitive detector cells 22.The planar digital detector 40 is positioned, as much as possible, in aperpendicular relation to a substantially central ray 17 of theplurality of rays 16 emitted by the x-ray source 14. The array 12 ofindividual photosensitive cells (pixels) 22 may be identified by theirposition according to column and row, however, the orientation of thecolumns and rows is arbitrary and does not limit the scope of anyembodiments disclosed herein. For clarity of description, it will beassumed that the rows extend horizontally and the columns extendvertically.

In one exemplary embodiment, the rows of photosensitive cells 22 may bescanned one or more at a time by electronic scanning circuit 28 so thatthe exposure data from the array 12 may be transmitted to electronicread-out circuit 30. Each photosensitive cell 22 may independently storea charge proportional to an intensity, or energy level, of theattenuated radiographic radiation, or x-rays, received and absorbed inthe cell. Thus, each photosensitive cell, when read-out, providesinformation defining a pixel of a radiographic image 24 that may bedigitally decoded and displayed by a digital monitor 26 for viewing by auser. An electronic bias circuit 32 is electrically connected to thedetector 12 to provide a bias voltage to each of the photosensitivecells 22.

Each of the bias circuit 32, the scanning circuit 28, and the read-outcircuit 30, may communicate with, via a wired or wireless connection, anacquisition control and image processing unit 34 that includes aprocessor and electronic memory (not shown) to control operations of thedetector 40 as described herein below, including circuits 30, 28 and 32,for example, by use of programmed instructions. The acquisition controland image processing unit 34 may also be used to control activation ofthe x-ray source 14 during a radiographic exposure, controlling an x-raytube electric current magnitude, and thus the fluence of x-rays in x-raybeam 16, and/or the x-ray tube voltage, and thus the energy level of thex-rays in x-ray beam 16.

The acquisition control and image processing unit 34 may transmit image(pixel) data to the monitor 26, based on the radiographic exposure datareceived from the array 12 of photosensitive cells 22. Alternatively,acquisition control and image processing unit 34 can process the imagedata, store raw or processed image data in local or remotely accessiblememory or export the image data.

With regard to direct detection, photosensitive cells 22 may eachinclude a sensing element sensitive to x-rays, i.e. it absorbs x-raysand generates an amount of charge carriers in proportion to a magnitudeof the absorbed x-ray energy, and a switching element that isselectively activated to read out the charge level of the correspondingsensing element. With regard to indirect detection, photosensitive cells22 may each include a sensing element sensitive to light rays in thevisible spectrum, i.e. it absorbs light rays and generates an amount ofcharge carriers in proportion to a magnitude of the absorbed lightenergy, and a switching element that is selectively activated to readthe charge level of the corresponding sensing element. A scintillator,or wavelength converter, is disposed over the light sensitive sensingelements to convert incident x-rays to visible light rays.

Examples of sensing elements used in sensing arrays 12 include varioustypes of photoelectric conversion devices (e.g., photosensors) such asphotodiodes (P-N or PIN diodes), photo-capacitors (MIS),photo-transistors or photoconductors. Examples of switching elementsused for signal read-out include MOS transistors, bipolar transistorsand p-n junction components.

FIG. 2 is a schematic diagram 240 of a portion of a two-dimensionalarray 12 for a digital radiographic detector 40. The array ofphotosensor cells 212 may include a number of a-Si:H n-i-p photodiodes270 and thin film transistors (TFTs) 271 formed as FETs each having gate(G), source (S), and drain (D) terminals. A plurality of gate drivercircuits 228 may be electrically connected to a plurality of gate lines283 which control a voltage applied to the gates of TFTs 271, aplurality of readout circuits 230 may be electrically connected to datalines 284, and a plurality of bias lines 285 may be electricallyconnected to a bias line bus or a variable bias reference voltage line232 which controls a voltage applied to the photodiodes 270. Chargeamplifiers 286 may be electrically connected to the data lines 284 toreceive signals therefrom. Outputs from the charge amplifiers 286 may beelectrically connected to a multiplexer 287, such as an analogmultiplexer, then to an analog-to-digital converter (ADC) 288, or theymay be directly connected to the ADC, to stream out the digital imagedata at desired rates. In one embodiment, the schematic diagram of FIG.2 may represent a digital radiographic detector 40 such as ahydrogenated amorphous silicon (a-Si:H) based indirect flat panelimager.

Incident X-ray photons, or x-rays, 16 are converted to optical photons,or light rays, by a scintillator, which light rays are subsequentlyconverted to electron-hole pairs, or charges, upon impacting the a-Si:Hn-i-p photodiodes 270. In one embodiment, an exemplary detector cell 22,222, which may be equivalently referred to herein as a pixel, mayinclude a photodiode 270 having its anode electrically connected to abias line 285 and its cathode electrically connected to the drain of TFT271. The bias reference voltage line 232 can control a bias voltage ofthe photodiodes 270 at each of the detector cells 22. The chargecapacity of each of the photodiodes 270 is a function of its biasvoltage and its capacitance. In general, a reverse bias voltage, e.g. anegative voltage, may be applied to the bias lines 285 to create anelectric field (and hence a depletion region) across the pn junction ofeach of the photodiodes 270 to enhance its collection efficiency of thecharges generated by incident light rays. The image signal representedby the array of photosensor cells 212 may be integrated by thephotodiodes while their associated TFTs 271 are held in a non-conducting(off) state, for example, by maintaining the gate lines 283 at anegative voltage via the gate driver circuits 228. The photosensor cellarray 212 may be read out by sequentially switching rows of the TFTs 271to a conducting (on) state by means of the gate driver circuits 228.When a row of the pixels 22 is switched to a conducting (“on”) state,for example by applying a positive voltage to the corresponding gateline 283, collected charge from the photodiode in those pixels may betransferred along data lines 284 and integrated by the external chargeamplifier circuits 286. The row may then be switched back to anon-conducting state, and the process is repeated for each row until theentire array of photosensor cells 212 has been read out. The integratedsignal outputs are transferred from the external charge amplifiers 286to an analog-to-digital converter (ADC) 288 using a parallel-to-serialconverter, such as multiplexer 287, which together comprise read-outcircuit 230. This digital image information may be subsequentlyprocessed by image processing system 34 to yield a digital image whichmay then be stored and immediately displayed on monitor 26, or displayedat a later time by accessing electronic memory containing the storedimage. The flat panel imager having an imaging array as described withreference to FIG. 2 is capable of both single-shot (e.g., static,radiographic) and continuous (e.g., fluoroscopic) image acquisition.

FIG. 3A shows a perspective view of a portable wireless DR detector 300according to one embodiment disclosed herein. FIG. 3B shows a portion ofa cross-section view along line A-A of the DR detector 300. As shown inFIGS. 3A-3B, the portable DR detector 300 may include a housingcomprising a top cover 312, and enclosure 314, enclosing at least ascintillator 320 and a two-dimensional imaging array 340, as describedherein.

In one embodiment, the DR detector 300 housing may include an enclosure314, having a top cover 312, attached thereto, made of material thatpasses x-ray flux 316 without significant attenuation. Scintillator 320may be under (e.g., directly connected to) the top cover 312, and theimaging array 340 may be directly under the scintillator 320. Readoutelectronics 328 may be co-planar with the imaging array 340, partiallybelow support member 324 or on a flexible connector therebetween. Thex-ray flux 316 may pass through the top panel cover 312, impinge uponscintillator 320 where stimulation by high-energy rays, or photons, inthe x-ray flux 316 causes the scintillator 320 to emit low energyphotons 332 as visible light rays.

The support member 324 can be included to securely and/robustly mountthe imager 240 and can further operate as a shock absorber betweencomponents therein and the enclosure 314. Device electronics requiredfor proper operation of the detector can be mounted within the enclosure314 and can include electronic components 328 (e.g., processors, FPGAs,ASICs, chips, etc.) that can be mounted on one or more separate and/orinterconnected circuit boards 326.

Exemplary embodiments described herein relate to methods and/or systemsof temperature compensation of the TFT transfer characteristics in adigital x-ray detector for the purpose of maintaining calibration over awide operating temperature range.

In the operation of the DR detector, or digital X-ray detector (DXD), itis advantageous to have the capability of operating over a widetemperature range without requiring pixel-by-pixel calibration orrecalibration for each monitored operating temperature. To minimizecalibration time and complexity, the pixel-by-pixel gain calibrationdata for sensitivity to X-ray exposure can be performed at a singletemperature (e.g., 25° C.) and stored in a table in electronic memoryfor later access by the image processing system 34. This gaincalibration at a single temperature is typically performed by exposingthe detector to a uniform magnitude X-ray exposure and measuring thedetector output divided by the X-ray exposure level to give a numericalmeasure of electrons per incident X-ray on a pixel-by pixel basis, whichdata may be stored on a pixel-by pixel basis. This single temperaturegain calibration data is used to adjust the detector output duringdetector operation to remove spatial variation in gain. In usage of aDXD, it is desirable for this single temperature gain calibration datato apply over a wider range of operating temperatures, for example, 15°C. to 35° C. However, it is well known that the TFT characteristics canchange significantly over even a 20° C. temperature change in the DXDoperating temperature. A significant TFT parameter for passive-pixel DXDoperation is the channel resistance R_(TFT)(V_(GS), −V_(T), V_(DS), T)between the source (S) and drain (D) of the TFT where V_(GS) is thevoltage between gate (G) and source (S), V_(T) is the TFT thresholdvoltage, V_(DS) is the voltage between source and drain and T is theoperating temperature. During charge transfer (read out) from thephotodiode to the dataline, the TFT gate can be switched to a largepositive voltage (such as +25V) in order to turn on the TFT while thesource can be held at 0V and the V_(DS) can be <1V. In these conditions,the TFT can be in the linear region of operation and the resistance isindependent of V_(DS) to first order. The TFT channel resistance candepend on the inverse of the electron mobility μ_(e)(T) as a function oftemperature (T). Electron movement in amorphous silicon can be describedby a hopping process in which the electron becomes trapped in shallowtraps in the amorphous silicon and then is thermally excited out of thetrap and transports a short distance under the influence of the electricfield between drain (D) and source (S) followed by capture by anothertrap. Since the hopping process is thermally activated, the mobility hasa strong temperature (T) dependence, with higher mobility at highertemperatures. In practice, it is found that the TFT resistance canchange by a factor of 2× or more over a 20° C. temperature change inoperating temperature of a DXD.

The change in TFT resistance has an effect on the accuracy of gaincalibration and the stored gain calibration data may be modifiedaccording to the monitored (detected) operating temperature. Chargetransfer from the photodiode to the dataline is limited by the chargetransfer time constant τ_(RC), which may be calculated as follows:τ_(RC) =R _(TFT)(V _(GS) ,T)*C _(PD)  (1)where R_(TFT) is the TFT channel resistance at gate (G) to source (S)voltage V_(GS) and temperature T, and C_(PD) is the photodiodecapacitance. For a typical digital X-ray detector, R_(TFT)(V_(GS)=25V,T=25° C.)=3 MΩ and C_(PD)(V_(PD)=4V, T=25° C.)=3 pF, for a transfer timeconstant τ_(RC)=9 μs. In general, the dependence of R_(TFT) on V_(DS)and the dependence of C_(PD) on bias and temperature are small and canbe neglected.

During operation, the DR detector is exposed to X-rays with the gates ofall of the TFT's in the off-state (V_(GS)<0). The detector is then readout by clocking each row of gates to a large positive voltage V_(GS)˜25Vto switch them into a conducting (on) state for a transfer time T_(TR).

For a photodiode exposed to X-rays to create a signal charge N₀electrons, the charge N_(sig) transferred from the photodiode to thedataline during the transfer time is given by:N _(sig) =N ₀*(1−exp(−T _(TR)/τ_(RC)))  (2)

For a typical DR detector, the transfer time T_(TR) is determined by thedesired readout time and is typically 20-50 μs. Both R_(TFT) and C_(PD)are not uniform across the detector area due to process variations inphotodiode thickness (which affects C_(PD)) and TFT channel length(which affects R_(TFT)). This spatial variation in R_(TFT) and C_(PD)can be reduced or removed by determining the detector gain calibrationat a single temperature and storing it for later compensation duringimage capture. However, over a 20° C. temperature range the approximate2× change in R_(TFT)(V_(GS), T) will cause the signal charge N_(sig) asread out from the data line to change, resulting in an inability toperform accurate calibration of the DR detector for the operatingextremes. This results in the spatial variation to become visible in theX-ray images taken over (e.g., at the extremes) the operatingtemperature range of a DXD.

Certain exemplary embodiments described herein may provide methods andapparatus to achieve accurate gain calibration over a wide temperaturerange in a DXD without the need to calibrate and recalibrate at multipletemperatures. Certain exemplary embodiments rely on (i) the dependenceof the R_(TFT)(V_(GS), T) on gate voltage V_(GS) and temperature T or(ii) on monitoring R_(TFT) or a variable related to R_(TFT) in paneloperation and adjusting one or more operating points of the panel toachieve a target value of T_(TR) and/or τ_(RC). In certain exemplaryembodiments disclosed herein, the temperature of the DR detector (e.g.,or at least one temperature sensitive parameter such as TFT resistanceor image lag) during detector readout can be monitored in the x-raydetector cassette and can be used by software in a detectormicro-controller in the detector cassette or in the image processor inthe detector console 34 to adjust, or modify, settings for detectoroperation. In several method embodiments described below, thetemperature dependence of either the TFT resistance R_(TFT)(V_(GS), T)or of the charge transfer time constant τ_(RC) can be measured duringdetector manufacturing (on a detector-by-detector basis) and can be usedin the detector adjustment. Such detector adjustment may be based oncalibration data stored in table form in electronic memory, whereinnumerical calibration data is associated with a particular temperaturein the table, for example, as shown in FIG. 4. In one embodiment,software may also be used to program a micro-controller in the DRdetector housing 312, 314, or the image processing system 34 tocalculate the calibration data on the fly using a currently monitoredtemperature as an input to stored calibration formulas. The storedcalibration formulas may be determined and stored in association withthe DR detector based on the TFT measurements initially performed duringmanufacture of the DR detector, and as described herein below.

-   -   1. Monitor the temperature and apply a uniform adjustment of        gain. In one exemplary embodiment, an average temperature        dependence of either the TFT resistance or the charge transfer        time constant is measured in manufacturing or is known for the        transistor process. The gain (e.g., photo-sensitivity in        electrons per photon) that can be measured on a pixel-by-pixel        basis is calibrated at a single temperature during detector        calibration (e.g., once per year). During detector operation,        the gain calibration table applied to all pixels would be        adjusted according to the measured temperature dependence of        R_(TFT) or τ_(RC). In this method embodiment, no changes in        detector operation are made based on the temperature; rather,        the calibration table used by the DR detector is adjusted by a        uniform factor depending on the temperature.    -   2. Measure the temperature dependence of charge transfer time        constant or of detector gain at manufacturing, modify the        accessed gain table on a pixel by pixel basis: In this exemplary        embodiment, the temperature dependence of either the detector        gain or of charge transfer time constant τ_(RC) is measured for        each pixel in the array at manufacturing and a map (e.g., or        parameter list) is stored as original calibration data. In        usage, the detector is re-calibrated at a single temperature        (typically 25° C.). The original calibration data with the        temperature dependence can then be applied to generate        (calculate) a new calibration data gain table. Since the TFT        resistance does not change over time, the original calibration        should remain valid as a basis for the life of the detector.    -   3. Monitor the image lag and adjust the gain table accordingly:        At lower operating DXD temperatures where charge transfer is        less complete (see equation (2) herein), a sizeable fraction of        the remaining charge will be read out in the next image        following readout of the exposed image. In this exemplary        embodiment, when the amount of image lag is monitored, then        adjustments can be made to the gain table to compensate.        Alternatively, the image lag can be measured on a pixel-by-pixel        basis as a function of temperature at detector manufacturing and        a temperature correction table generated to be applied to each        gain table depending on the temperature measured in the cassette        at the time of operation. In another embodiment, the image lag        (e.g., image lag table) can be generated by using electrical        injection prior to (or following) exposure. In this embodiment,        signal charge can be electrically written to the pixel by a fill        pulse in which the diode bias is reduced by a small voltage        (ΔV_(PD)=˜0.5V) during a first readout and restored to its        nominal voltage during a second readout. During the filling        pulse, a signal charge with N electrons where N=C_(PD)ΔV_(PD) is        written to the diode. The first frame after the fill pulse        (signal frame) can be and/or the second frame after the fill        pulse (lag frame) can be used to generate a correction to the        gain table.    -   4. Monitor temperature and a just transfer time T_(TR): Since        the charge transfer from the photodiode to the dataline depends        only on the ratio T_(TR)/τ_(RC), if the temperature dependence        of R_(TFT)(V_(GS), T) is measured, then the transfer time can be        adjusted based on the measured (monitored) temperature in the        cassette. In this exemplary embodiment, the micro-controller in        the DR detector cassette can sense the temperature prior to        image capture, and it, or the image processing 34, would access        the calibration data table and accordingly may adjust the timing        of the gate driver circuits 228. For example, the transfer time        at 15° C. might be set to 50 μs while the transfer time at        35° C. might be set to 25 μs in the calibration data table, in        order to maintain a constant ratio of T_(TR)/τ_(RC).    -   5. Monitor temperature and TFT adjust gate voltage: The TFT        resistance, and thereby the charge transfer time constant        τ_(RC), depends on the gate voltage V_(GS), with a lower        resistance at a higher gate voltage. In this exemplary        embodiment when the gate voltage dependence of the TFT        resistance is measured, then the gate voltage can be adjusted in        order to maintain a constant ratio T_(TR)/τ_(RC). For example,        if the micro-controller in the cassette measures a temperature        of 15° C., it, or the image processing 34, would access the        calibration data table and accordingly set the gate driver        on-voltage to 30V while if the micro-controller measures 35° C.,        it would set the gate driver on-voltage to 20V to compensate for        the temperature dependence of R_(TFT).    -   6. Monitor a test circuit to sense TFT resistance or a parameter        sensitive to TFT resistance and adjust either transfer time or        TFT gate voltage. In this exemplary embodiment, a test circuit        can be included on the detector to monitor the resistance of a        test TFT prior to detector readout, then the micro-controller        can modify an operating parameter such as transfer time or gate        on-voltage to compensate. Such a circuit, for example, might        include multiple TFT's of the same dimensions as those used as        pixel switches in the array biased at a known value of V_(DS).        The micro-controller, or image processing 34, could set the gate        voltage to a value used in detector operation and measure the        current output I_(DS), which would be inversely proportional to        the TFT resistance. In an alternative embodiment, the        micro-controller, or processor, could adjust V_(GS) to achieve a        target value of I_(DS). The on-voltage of the gate drivers could        then be set to the same voltage during detector readout. In one        embodiment, the test circuit may be outside of the imaging        array, while in another embodiment the test circuit may be        fabricated within the imaging array (e.g., dispersed or        contiguous).    -   7. Measure the signal charge and/or lag charge following        electrical injection before capture of the radiographic image        and adjust operating parameters accordingly: As described in #3        above, electrical injection can be used to write a known signal        charge to each pixel and monitor the signal charge in the first        image frame after electrical injection (signal frame) or the        second or following image frames (lag frames). In this exemplary        embodiment, the remaining charge information in lag frames can        be used to adjust either the transfer time T_(TR) or the gate        on-voltage V_(GS). In one embodiment, a process could be        performed iteratively with adjustment of T_(TR) or V_(GS) until        the signal and/or lag frames match a pre-determined stored        profile.

In the first three exemplary embodiments described above, no changes tothe operating parameters for a DXD are made based on the temperature.Rather, adjustment to the calibration table used for detector uniformitycorrection are made based on the measured temperature at the time ofoperation, i.e., at approximately the time of x-ray exposure and imagecapture. It will be apparent that many algorithms of various levels ofcomplexity can be used to compensate the calibration data gain table foruse at the actual temperature of operation. In exemplary embodiments #4through #7 above, one or more of the operating parameters for a DXD aremade based on the temperature at the time of operation or on othercalibration methods (such as measurement of response to electricalinjection). Other measurements or adjustments of other operatingparameters or other post-processing of the images can be used that willyield similar results.

In summary, in a DR detector it is advantageous to have the capabilityof operating over a wide temperature range without requiringpixel-by-pixel gain calibration for each operating temperature. In orderto minimize calibration time and complexity, the pixel-by-pixel gaincalibration for sensitivity to X-ray exposure is performed at a singletemperature (such as 25° C.). This calibration is typically performed byexposing the detector to a uniform (or known) X-ray exposure andmeasuring the detector output, subtracting the detector output in theabsence of X-ray exposure (offset-corrected exposed image) divided bythe X-ray exposure (in units such as milli-Roengten (mR) to giveelectrons per incident X-ray on a pixel-by pixel basis. The gain is theelectrical output of the detector (in analog or digital form) divided bythe X-ray input (in units such as milli-Roengten (mR)) for a standardset of X-ray exposure condition. The absolute value of the detector gaindepends on parameters such as the number and wavelength of the visiblephotons per unit area emitted by the scintillator for the standard X-rayexposure, the optical coupling between scintillator and the array, thephoto-sensing element area, the quantum efficiency vs. wavelength forthe photo-sensing element in the pixel, the percentage of chargetransferred from the photodiode to the readout circuit, and theconversion factor of charge to detector output signal. All of theseparameters except the charge transfer efficiency (percentage of chargetransferred from the photosensor to the readout circuit) are relativelyindependent of temperature over normal operating temperature for DRdetectors. The charge transfer efficiency, however, is stronglytemperature dependent and accounts for the majority of the temperaturesensitivity of the gain.

The present application contemplates methods and program products on anycomputer readable media for accomplishing its operations. Exemplaryembodiments according to the present application can be implementedusing an existing computer processor, or by a special purpose computerprocessor incorporated for this or another purpose or by a hardwiredsystem. Also known in the art are digital radiographic imaging detectorsthat utilize an array of pixels comprising an X-ray absorbingphotoconductor, such as amorphous Selenium (a-Se), and a readoutcircuit. Since the X-rays are absorbed in the photoconductor, noseparate scintillating screen is required.

It should be noted that while the present description and examples areprimarily directed to radiographic medical imaging of a human or othersubject, embodiments of apparatus and methods of the present applicationcan also be applied to other radiographic imaging applications. Thisincludes applications such as non-destructive testing (NDT), for whichradiographic images may be obtained and provided with differentprocessing treatments in order to accentuate different features of theimaged subject.

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method, or computer program product.Accordingly, an embodiment of the present invention may be in the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, and other suitableencodings) or an embodiment combining software and hardware aspects thatmay all generally be referred to herein as a “circuit” or “system.”Furthermore, the present invention may take the form of a computerprogram product embodied in a computer-readable storage medium, withinstructions executed by one or more computers or host processors. Thismedium may comprise, for example: magnetic storage media such as amagnetic disk (such as a hard drive or a floppy disk) or magnetic tape;optical storage media such as an optical disc, optical tape, or machinereadable bar code; solid state electronic storage devices such as solidstate hard drives, random access memory (RAM), or read only memory(ROM); or any other physical device or medium employed to store acomputer program. The computer program for performing the method of thepresent invention may also be stored on computer readable storage mediumthat is connected to a host processor by way of the internet or othercommunication medium.

Those skilled in the art will readily recognize that the equivalent ofsuch a computer program product may also be constructed in hardware. Thecomputer-usable or computer-readable medium could even be paper oranother suitable medium upon which executable instructions are printed,as the instructions can be electronically captured, via, for instance,optical scanning of the paper or other medium, then compiled,interpreted, or otherwise processed in a suitable manner, if necessary,and then stored in a computer memory. In the context of this document, acomputer-usable or computer-readable medium may be any medium that cancontain, store, communicate, propagate, or transport computerinstructions for use by, or in connection with, an instruction executionsystem, apparatus, or device.

While the invention has been illustrated with respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theinvention can have been disclosed with respect to only one of severalimplementations, such feature can be combined with one or more otherfeatures of the other implementations as can be desired and advantageousfor any given or particular function. The term “at least one of’ is usedto mean one or more of the listed items can be selected. The term“about” indicates that the value listed can be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated embodiment. Finally, “exemplary” indicatesthe description is used as an example, rather than implying that it isan ideal. Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A method of operating a digital radiographicdetector having a plurality of pixels, the method comprising:determining and electronically storing a gain calibration tableapplicable to the plurality of pixels in the detector at a firsttemperature; capturing radiographic image data using the plurality ofpixels in the detector at a second temperature different from the firsttemperature; adjusting the gain calibration table based on the secondtemperature; applying the adjusted gain calibration table to theradiographic image data captured at the second temperature; and readingout the radiographic image data from the detector after the step ofapplying the adjusted gain calibration table thereto.
 2. The method ofclaim 1, further comprising predetermining a channel resistance factor,wherein the channel resistance factor is associated with a temperaturedependency of channel resistance of each of the plurality of pixels inthe detector, and wherein the step of adjusting comprises uniformlyadjusting the gain calibration table using the channel resistancefactor.
 3. The method of claim 2, wherein the channel resistance factorand the gain calibration table are stored in an electronic memoryaccessible by the detector.
 4. The method of claim 2, further comprisingdetermining the channel resistance factor according to a temperatureformula associated with the detector.
 5. The method of claim 3, furthercomprising determining the channel resistance factor associated with thedetector at the first temperature at approximately the time ofmanufacture of the detector.
 6. The method of claim 1, furthercomprising predetermining a time constant factor of the plurality ofpixels in the detector and storing it in an electronic memory accessibleby the detector, wherein the time constant factor is associated with atemperature dependency of a time constant of each of the plurality ofpixels in the detector, and wherein the step of adjusting comprisesadjusting the gain calibration table using the time constant factor. 7.The method of claim 1, further comprising predetermining an image lagfactor of the plurality of pixels in the detector and storing it in anelectronic memory accessible by the detector, wherein the image lagfactor is associated with a temperature dependency of image lag of eachof the plurality of pixels in the detector, and wherein the step ofadjusting comprises adjusting the gain calibration table using the imagelag factor.
 8. A method of operating a digital radiographic detector,the method comprising: determining a pixel gate voltage associated withthe detector at a first temperature; storing the pixel gate voltageassociated with the detector at the first temperature in an electronictable accessible by the detector; capturing a radiographic image usingthe detector at a second temperature; determining a pixel gate voltageassociated with the detector at the second temperature; and reading outthe radiographic image data from the detector at the determined pixelgate voltage associated with the detector at the second temperature. 9.The method of claim 8, further comprising: capturing a radiographicimage using the detector at the first temperature; and reading out theradiographic image data from the detector at the determined pixel gatevoltage associated with the detector at the first temperature.
 10. Themethod of claim 8, further comprising determining the pixel gate voltageaccording to a temperature formula associated with the detector.
 11. Themethod of claim 10, further comprising increasing the pixel gate voltagein response to a decreasing temperature of the detector according to thetemperature formula associated with the detector.
 12. The method ofclaim 6, further comprising using the time constant factor as a commonfactor for uniformly adjusting the gain calibration table.
 13. Themethod of claim 6, further comprising determining the time constantfactor according to a temperature formula associated with the detector.14. The method of claim 6, further comprising determining the timeconstant factor of the plurality of pixels in the detector at the firsttemperature at approximately the time of manufacture of the detector.15. The method of claim 6, further comprising using the time constantfactor as a common factor for uniformly adjusting the gain calibrationtable.
 16. The method of claim 7, further comprising determining theimage lag factor according to a temperature formula associated with thedetector.
 17. The method of claim 7, further comprising determining theimage lag factor of the plurality of pixels in the detector at the firsttemperature at approximately the time of manufacture of the detector.18. The method of claim 7, further comprising using the image lag factoras a common factor for uniformly adjusting the gain calibration table.